Yes, you can run luminous on Trusty; one of my clusters is currently Luminous/Bluestore/Trusty as I've not had time to sort out doing OS upgrades on it. I second the suggestion that it would be better to do the luminous upgrade first, retaining existing filestore OSDs, and then do the OS upgrade/OSD recreation on each node in sequence. I don't think there should realistically be any problems with running a mixed cluster for a while but doing the jewel->luminous upgrade on the existing installs first shouldn't be significant extra effort/time as you're already predicting at least two months to upgrade everything, and it does minimise the amount of change at any one time in case things do start going horribly wrong.
Also, at 48 nodes, I would've thought you could get away with cycling more than one of them at once. Assuming they're homogenous taking out even 4 at a time should only raise utilisation on the rest of the cluster to a little over 65%, which still seems safe to me, and you'd waste way less time waiting for recovery. (I recognise that depending on the nature of your employment situation this may not actually be desirable...)
Rich
I also agree with this approach.... we actually did the reverse, updated OS on all nodes from precise/trusty to xenial while cluster was still running hammer. the only thing that we had to fiddle with was init (ie. no systemd files provided with hammer), but you can write basic script(s) to start/stop all osds manually. this was ok for us, particularly since we didn't intend to run that state for a long period, and eventually upgraded to jewel and soon to be luminous. In your case, since trusty is supported in luminous I don't think you would have any trouble with this?
Rafael Lopez
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